Question

Inside a nerve cell at rest, [Na⁺] is _____ and [K⁺] _____ relative to the concentrations seen outside the cell.

low; low

low; equal

high; high

high; low

low; high

Answer 1

Inside a nerve cell at rest, the [Na⁺] is low and [K⁺] is high relative to the concentrations seen outside the cell.

This is due to the net movement of ions accross the membrane with the concentration gradient. The cell membrane is more permeable to K⁺ ions compared to Na⁺ ions. So K⁺ ions accumulates more inside the cell where the Na⁺ ions accumulated outside the cell. The cell possesses K⁺ and Na⁺ leakage channels that allow the two cations to diffuse down their concentration gradient. However, the neurons have far more potassium leakage channels than sodium leakage channels. Therefore, potassium diffuses out of the cell at a much faster rate than sodium leaks in. More cations leaving the cell than entering it causes the interior of the cell to be negatively charged relative to the outside of the cell. The actions of the sodium-potassium pump help to maintain the resting potential, once it is established. Recall that sodium-potassium pumps bring two K⁺ ions into the cell while removing three Na+ ions per ATP consumed.